Environmentally-resistant turbine blade tip

ABSTRACT

A family of environmentally-resistant alloys is provided which are suitable for forming a blade tip for a turbine blade of a gas turbine engine. The blade tip alloys preferably have a chemical composition of, in weight percent, about 14 to about 18 percent chromium, about 6.45 to about 6.95 percent aluminum, about 9.75 to about 11.45 percent cobalt, about 5.95 to about 6.55 percent tantalum, about 1.85 to about 2.35 percent rhenium, about 0.05 to about 1.75 percent hafnium, about 0.006 to about 0.03 percent zirconium, about 0.02 to about 0.11 percent carbon, up to about 1.1 percent silicon, up to about percent 0.01 percent boron, with the balance being nickel and typical impurities.

This application is a continuation of application Ser. No. 08/290,662filed Aug. 15, 1994, abandoned.

The present invention relates to turbine blades used in the highpressure turbine section of a gas turbine engine, in which a protectivealuminide or overlay coating is used to enhance the hot corrosion andoxidation resistance of the turbine blades. More particularly, thisinvention relates to such a turbine blade having a blade tip which isalloyed to be resistant to hot corrosion and oxidation, andcharacterized by suitable mechanical properties such as high stressrupture, while also being capable of being reliably fused to the blade.

BACKGROUND OF THE INVENTION

The operation of axial flow gas turbine engines involves the delivery ofcompressed air to the combustion section of the engine where fuel isadded to the air and ignited, and thereafter delivered to the turbinesection of the engine where a portion of the energy generated by thecombustion process is extracted by a turbine to drive the enginecompressor. Accordingly, the efficiency of gas turbine engines isdependent in part on the ability to minimize leakage of compressed airbetween the turbine blades and the shroud of the engine's turbinesection. To minimize the gap between the turbine blade tips and theshroud, turbine blades often undergo a final rotor grind such that theturbine rotor assembly closely matches its shroud diameter. As a result,some degree of rubbing with the shroud typically occurs during operationdue to manufacturing tolerances, differing rates of thermal expansionand dynamic effects.

Turbine blades alloys are primarily designed to meet mechanical propertyrequirements such as creep rupture and fatigue strengths. However, manyturbine engines must operate under conditions which promote hotcorrosion and oxidation of the turbine blades formed from such alloys.Therefore, to enhance their environmental resistance, an aluminide oroverlay coating is often applied to the blades in order to provide aprotective and adherent layer of alumina scales. However, theabove-noted machining and rubbing to which the blades are subjectedoften results in the removal of the aluminide or overlay coating at theblade tips. As a result, the underlying blade material is exposed,leading to corrosion and/or oxidation that causes tip recession orfailure, which potentially leads to performance losses due to higherleakage between the blades and the shroud.

From the above, it can be appreciated that both new and worn turbineblades could benefit from being equipped with blade tips which arealloyed to be inherently resistant to oxidation and hot corrosion, suchthat removal of the blade's aluminide or overlay coating at the bladetip would not effect the environmental resistance of the blade.

While blade tip repair methods are known in the art, as evidenced byU.S. Pat. No. 4,808,055 to Wertz et al., such methods have notidentified blade tip alloys which satisfy the mechanical andenvironmental properties required for operation in a gas turbine engine,while also being capable of being reliably bonded to the turbine blade.

In particular, prior art blade tip alloys which exhibit desirablemechanical properties, such as high temperature stress rupture life, andenvironmental properties, such as resistance to hot corrosion andoxidation, have been prone to microcracking during deposition onto theturbine blade. Conversely, other prior art alloys have been identifiedwhich exhibit adequate tungsten inert gas (TIG) welding or laser fusingcharacteristics, but do not have the requisite mechanical and/orenvironmental properties.

Accordingly, it would be advantageous to provide an improved blade tipalloy for turbine blades of gas turbine engines, in which the blade tipalloy is characterized by suitable mechanical properties such as hightemperature stress rupture life and desirable environmental propertiessuch as resistance to oxidation and hot corrosion, while further havingdesirable weldability characteristics.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a turbine rotorblade whose blade tip is formed from an environmentally-resistant alloywhich is highly resistant to hot corrosion and oxidation.

It is a further object of this invention that such an alloy exhibitsuitable mechanical properties, such as high temperature stress rupturelife.

It is another object of this invention that such an alloy be capable ofbeing readily and reliable fused to the tip of the turbine blade, so asto minimize structural flaws such as microcracking.

In accordance with a preferred embodiment of this invention, these andother objects and advantages are accomplished as follows.

According to the present invention, there is provided a family ofenvironmentally-resistant alloys which are suitable for forming a bladetip for a turbine blade of a gas turbine engine. The blade tip alloys ofthis invention preferably have a chemical composition of, in weightpercent, of about 14 to about 18 percent chromium, about 6.45 to about6.95 percent aluminum, about 9.75 to about 11.45 percent cobalt, about5.95 to about 6.55 percent tantalum, about 1.85 to about 2.35 percentrhenium, about 0.5 to about 1.75 percent hafnium, about 0.006 to about0.03 percent zirconium, about 0.02 to about 0.11 percent carbon, up toabout 1.1 percent silicon, up to about 0.01 percent boron, with thebalance being nickel and typical impurities. As such, the blade tipalloys of this invention differ from that of alloys typically used toform a conventional turbine blade.

A preferred method by which the above alloys of this invention are usedto form a blade tip on a turbine blade involves the steps of providingan alloy of this invention in the form of a powder, and then forming ablade tip by melting and fusing the powder alloy to the tip of theturbine blade. For certain applications, the blade tip may be formed bydepositing the molten powder alloy utilizing a weld stitching patterncharacterized by a weld path which traverses the length of the chord ofthe turbine blade while cyclicly traversing the width of the turbineblade, generally in a zig-zag manner. The rate of deposition ischaracterized by an instantaneous velocity V_(i) along the weld path andan average velocity V_(a) by which the weld path advances in thedirection of the chord length of the turbine blade. Importantly, it hasbeen found that maintaining the ratio of V_(i) to V_(a) at about 3:1 toabout 12:1 significantly reduces stresses created during deposition, andthereby minimizes microcracking of the blade tip. Preferably, the bladetip is generated by depositing the molten alloy powder in multiplepasses. In accordance with this invention, the forming step may be usedto generate a near-net shape blade tip, or a subsequent machining stepmay be employed to generate the final preferred shape of the blade tip.

In accordance with the present invention, the relatively narrowcompositional range of the blade tip alloy yields a turbine blade tipcharacterized by suitable mechanical properties such as high temperaturestress rupture life, desirable environmental properties such asresistance to oxidation and hot corrosion, and desirable weldabilitycharacteristics. As such, the removal of a blade's aluminide or overlaycoating at its blade tip will not significantly effect the environmentalresistance of the blade. An additional advantage of the presentinvention is that both new and worn turbine blades can benefit frombeing equipped with blade tips formed in accordance with this invention.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages of this invention will become moreapparent from the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a perspective view of a high pressure turbine blade equippedwith a blade tip in accordance with this invention;

FIGS. 2 and 3 are schematic representations of a suitable laser weldfacility for forming the blade tip of this invention;

FIG. 4 is a schematic representation of a preferred weld patternutilized to form the blade tip of FIG. 1;

FIG. 5 is a cross-sectional view of a blade tip which may be generatedwith the weld pattern of FIG. 4;

FIGS. 6 and 7 are cross-sectional views of squealer blade tipconfigurations which may be machined from the blade tip of FIG. 5;

FIG. 8 is a perspective view of a turbine blade equipped with a squealerblade tip configuration; and

FIGS. 9 and 10 are cross-sectional views of alternative blade tipconfigurations for the turbine blade of FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an improved blade tip for turbine bladesused in gas turbine engines, and particularly turbine blades used in thehigh pressure turbine section of axial flow gas turbine engines. Arepresentative turbine blade 10 is illustrated in FIG. 1, and is shownequipped with a blade tip 12 formed in accordance with one embodiment ofthis invention. The blade 10 is illustrative of the general geometry ofturbine blades known in the art. Viewed from the blade tip end of theblade 10, the blade 10 is characterized by a chord length which extendsin the direction of the blade's longest cross-sectional dimension, andby a chord width which is transverse to the chord length.

The blade 10 is preferably formed from a suitable high temperaturematerial, such as an appropriate nickel-based superalloy of the typeknown in the art, and may be cast as single crystal or directionallysolidified casting to promote the high temperature properties of theblade 10.

In accordance with this invention, a family of alloys has beenidentified which will yield a blade tip 12 having the requiredmechanical and environmental properties and desirable weldabilitycharacteristics for survival in the more hostile environments endured bygas turbine engines. Due to the mechanical properties required of theremainder of the turbine blade 10, the preferred alloys of thisinvention differ compositionally from that of alloys used to form theturbine blade 10. More specifically, the alloys suitable for the blade10 have the requisite mechanical properties such as creep rupture andfatigue strength, while the family of alloys of this invention possessthe required hot corrosion, oxidation and stress rupture propertiesspecific to the tip portion of the blade 10. As shown in FIG. 1, theblade tip 12 forms the entire tip of the blade 10, and generally has athickness on the order of about 0.010 to about 0.100 inch (about 0.25 toabout 2.5 millimeters) in order to allow for continued environmentalprotection of the tip even if the tip has been partially removed duringfinal grind or after significant tip rubbing in service.

The alloys of this invention preferably fall within the followingcompositional range, noted in weight percent:

TABLE I

about 14 to about 18 chromium;

about 6.45 to about 6.95 aluminum;

about 9.75 to about 11.45 cobalt;

about 5.95 to about 6.55 tantalum;

about 1.85 to about 2.35 rhenium;

about 0.05 to about 1.75 hafnium;

about 0.006 to about 0.03 zirconium;

about 0.02 to about 0.11 carbon;

up to about 1.1 silicon;

up to about 0.01 boron;

balance nickel and typical impurities.

The above alloying range is a modified chemistry of an adherent coatingalloy disclosed in U.S. Pat. No. 5,316,866 to Goldman et al., assignedto the assignee of this invention. While the adherent coating alloytaught by Goldman et al. exhibited excellent resistance to oxidation andcorrosion, it did not have optimal weldability characteristics.Accordingly, the alloy compositions within the above ranges wereparticularly alloyed to be optimal for forming the blade tip 12 using aneconomical welding operation suitable for manufacturing conditions.

A suitable welding facility for purposes of this invention is disclosedin U.S. Pat. No. 5,160,822 to Aleshin, assigned to the assignee of thisinvention, and is schematically illustrated in FIGS. 2 and 3. The methodtaught by Aleshin utilizes laser fusing techniques, powder alloys andcomputerized numerical control of a target blade. As illustrated in FIG.2, such a facility includes a laser 14, an enclosed beam delivery 16,laser focusing optics 18, a part positioning system 20, a vision systemfor part location and laser path control 22, a preheat box 24, and apowder feed system 26. The working and coordination of the individualparts of the facility are controlled through a computerized systemcontroller 34. FIG. 3 shows in greater detail the preheat box 24, ablade 10 to be processed, quartz lamps 28, a powder feed line 30extending from the powder feed system 26, and a laser beam tube 32 whichdirects a laser beam at the tip of the blade 10. In a conventionalmanner, the powder enters the laser beam in close proximity to the blade10 as it is manipulated to cause melting and weld build-up.

In accordance with Aleshin, the blade tip 12 of this invention can beformed using a weld stitch pattern illustrated in FIG. 4. Asillustrated, the preferred weld stitch pattern is characterized by aweld path 36 which traverses the chord of the turbine blade 10 whilecyclicly traversing the width of the turbine blade 10, generally in azig-zag manner. The rate of deposition is characterized by aninstantaneous velocity V_(i) along the weld path and an average velocityV_(a) in the direction which the weld path advances along the chordlength of the turbine blade. Importantly, the V_(i) to V_(a) ratio mustbe maintained at about 3:1 to about 12:1 in order to significantlyreduce stresses created during deposition, and thereby minimizemicrocracking of the blade tip 12 formed from one of the alloys of thisinvention.

Preferably, the blade tip 12 is generated by depositing the molten alloypowder in multiple passes, as represented by the multiple layers shownin the cross-sectional view of FIG. 5. Laser power levels for thisprocess preferably provide a power density order of magnitude of about10⁴ watts per square centimeter. Particularly desirable results areachieved by utilizing gradually increasing laser power during thedeposition of the first several weld layers, until a preferred powerlevel is attained and thereafter maintained for the remainder of themultiple passes, while employing a relatively low powder feed rate, onthe order of about 2 to about 10 grams per minute, during the firstseveral passes and thereafter increased to about 3 to about 30 grams perminute for the remainder of the multiple passes. This technique has beenfound to assure proper fusion with the alloy of the turbine blade 10without excessive heat. Additional weld process parameters required mayvary based on blade alloy and geometry, but a preferredcomponent-specific process can be readily identified by one skilled inthe art in view of the teachings of Aleshin.

In accordance with this invention, a near-net shape blade tip 12 can begenerated with the preferred laser fusing technique, or a subsequentmachining step may be employed to generate the final shape of the bladetip 12. Alternative configurations which may be machined from the solidblade tip 12 shown in FIGS. 1 and 5 are illustrated in FIGS. 6 and 7,which depict blade tips in the form of squealer tips, as they arecommonly referred to in the art. The blade tip 12 may be machined fromthe blade tip 12 shown in FIG. 5 to leave only squealers 38 at theperimeter of the blade tip 12, as shown in FIG. 7, or a portion 40 ofthe blade tip 12 may be left intact with the blade 10 as shown in FIG.6.

Alternatively, the squealers 38 may be formed with the laser fusingtechnique at near-net shape, as represented by FIG. 8. With thistechnique, the blade tip 12 is generated by depositing the molten alloypowder in multiple passes along a weld path which repeatedly traces theairfoil contour, in a manner similar to that discussed with respect toFIG. 5. An advantage to this approach is that the amount of weldmaterial which must be machined is reduced, but the resulting blade tip12 is more prone to weld and heat affected zone cracking from residualstresses developed from the more complex weld bead contour, in that theweld stitch pattern of FIG. 4 cannot be utilized. However, this type ofweld process may be preferable for tips formed on blades formed fromcertain alloys and/or with certain geometries. As illustrated in FIG. 9,a new blade 10 can be equipped with squealers 38, or a blade 10 withpre-existing but worn squealers 42 can be repaired with new squealers 38as shown in FIG. 10.

While the laser fusing technique of Aleshin is preferred, it isforeseeable that similar results can be achieved using tungsten inertgas (TIG) or plasma transferred arc (PTA) weld methods using a wire orpowder of the blade tip alloys of this invention. Other alternativemethods, such as detonation gun (D-gun), hyper velocity oxygen/fuel(HVOF), high energy plasma or vacuum plasma spray (VPS; also referred toas low pressure plasma spray (LPPS)) thermal spray methods may also besuccessfully used to form suitable blade tips with the alloys of thisinvention, though these processes may require additional steps, such asheat treatments or hot isostatic pressing, in order to achievecomparable stress rupture lives.

Other potentially suitable techniques for generating a blade tip 12 aredisclosed in U.S. Pat. Nos. 4,305,792, 4,789,441 and 4,810,334, whichinvolve a composite plating process which entraps fine powder particlesof the alloying ingredients in an electroplated nickel matrix, andsubsequently diffusion heat treats the plated deposit to develop thedesired mechanical and environmental properties. Again, however, variousheat treatment modifications may be required for enhancing finalmechanical and/or environmental properties.

The above-noted preferred family of alloys of this invention wereidentified through a series of tests. One finding of preliminary testingwas that the segregation of yttrium to the grain boundaries duringwelding promoted microcracking problems associated with thenickel-yttrium eutectic. Accordingly, and contrary to the coating alloytaught by Goldman et al., emphasis was placed on compositions with nointentional yttrium content and attention to those elements which affectgrain boundaries and environmental properties of superalloy-typecoatings. In addition, the compositions were alloyed to containrelatively low levels of boron to promote their weldability. Moregenerally, the elements were selected and varied with the objective ofdetermining alloys which could increase the creep rupture strength ofthe welded blade tip 12 in order to allow a thickness of up to about0.150 inch (about 38 millimeters), to minimize weld microcracking, andfor improved hot corrosion and oxidation properties.

In accordance with the above, an experimental range was established foreach element, and eighteen castings of alloy compositions within theexperimental ranges were prepared and tested to determine their solidusand liquidus temperatures and longitudinal and transverse (lateral)stress rupture lives at about 2100° F. and 2 ksi. Four of thecompositions were then selected for full evaluation as laser weldeddeposits from powder alloy. The experimental range and the compositionranges of the four selected alloys, in weight percent, and test resultsfor the four alloys were as follows:

                                      TABLE II                                    __________________________________________________________________________               ALLOY                                                                         Exp.  A     B    C     D                                           __________________________________________________________________________    CHROMIUM   15-18 18    15   15    15                                          ALUMINUM   6.5 nom.                                                                            6.5   6.5  6.5   6.5                                         COBALT     10 nom.                                                                             10    10   10    10                                          TANTALUM   6 nom.                                                                              6     6    6     6                                           RHENIUM    2 nom.                                                                              2     2    2     2                                           HAFNIUM    0.05-1.5                                                                            1     1.5  1.5   0.5                                         SILICON    0-1   0     1    0     0.5                                         ZIRCONIUM  0.006-0.03                                                                          0.03  0.015                                                                              0.015 0.015                                       CARBON     0.02-0.1                                                                            0.1   0.02 0.1   0.05                                        BORON        0-0.01                                                                            0.004 0.004                                                                              0     0.01                                        NICKEL     bal.  bal.  bal. bal.  bal.                                        Solidus (°F.)                                                                           2326  2203 2366  2362                                        Liquidus (°F.)                                                                          2456  2437 2458  2468                                        Stress Rupture (hrs):                                                         Longitudinal     121.4 185.2                                                                              >400  >400                                        Lateral          61.0  85.15                                                                              201.3 149.2                                       __________________________________________________________________________

Generally, Alloy A was characterized as having high chromium, a moderatesolidus temperature and good rupture life, Alloy B was characterized ashaving a low solidus temperature and a wide melting range, Alloy C wascharacterized as having an exceptional stress rupture life and a highsolidus temperature, and Alloy D was characterized as having anexcellent stress rupture life.

Powders suitable for laser fusing were then prepared of Alloys A-D. Fromthe powders, pin specimens were made for environmental testing, andblade tips were formed on nickel superalloy turbine blades formetallographic evaluation. The environmental testing involved a hotcorrosion test conducted at about 1700° F., 5 ppm sea salt, with depthof penetration being measured after about 200 hours, and an oxidationtest conducted at about 2150° F., Mach 1, with depth of penetrationbeing measured after about 200 hours. The results were as follows:

                  TABLE III                                                       ______________________________________                                                HOT CORROSION                                                                              OXIDATION                                                ______________________________________                                        Alloy A   about 0.000 inch                                                                             about 0.0023 inch                                    Alloy B   about 0.000 inch                                                                             about 0.0012 inch                                    Alloy C   about 0.000 inch                                                                             about 0.0012 inch                                    Alloy D   about 0.000 inch                                                                             about 0.0023 inch                                    ______________________________________                                    

The above results indicated that all of the alloys were highly resistantto hot corrosion and oxidation, even without a protective aluminide oroverlay coating, and that Alloys B and C exhibited particularlyexcellent environmental resistance. Blade tips 12 were then formed usingthe preferred laser fusing technique described above and examinedmetallographically, with the result being that all of the alloysexhibited minimal microcracking, with Alloys B and C having a lowerlevel of microcracking than Alloys A and D.

From the above, Alloys B and C were identified as being preferred bladetip compositions for purposes of this invention. Generally, each exhibitimproved inherent environmental resistance so as to allow removal ofmaterial and/or a blade's protective coating without degrading theenvironmental properties of the blade. Simultaneously, Alloys B and Cenable the use of the laser fusing technique described above, whichprovides for an economical process that yields minimal weldmicrocracking due to the unique combination of alloy chemistry and laserweld technique. Alloy B is believed to be more compatible with bladealloys having lower melting points due to its lower solidus temperature,while Alloy C is believed to be more compatible with blade alloys havinghigher melting points.

Further assessment of Alloys B and C using known Taguchi statisticalmethods generated preferred chemistries and tolerances, in weightpercent, for Alloys B and C, as follows.

                  TABLE IV                                                        ______________________________________                                                ALLOY                                                                         B      C            Tolerance (±)                                  ______________________________________                                        CHROMIUM  16.00    15.60        1.00                                          ALUMINUM  6.7      6.7          0.25                                          COBALT    10.50    10.70        0.75                                          TANTALUM   6.20     6.30        0.25                                          RHENIUM    2.05     2.15        0.20                                          HAFNIUM    1.60     1.60        0.15                                          SILICON    0.90         0.2 max 0.20  (Alloy B)                               ZIRCONIUM  0.018    0.018       0.01                                          CARBON     0.030    0.100       0.01                                          BORON      0.005         0.002 max                                                                            0.002 (Alloy B)                               NICKEL    bal.      bal.                                                      ______________________________________                                    

From the above, it can be seen that an advantage of the presentinvention is that the relatively narrow preferred compositional range ofthe blade tip alloy yields a turbine blade tip characterized bymechanical properties such as high temperature stress rupture life,desirable environmental properties such as resistance to oxidation andhot corrosion, and desirable weldability characteristics. As such, for aturbine blade equipped with the blade tip of this invention, the removalof an aluminide or overlay coating at the blade tip will notsignificantly effect the environmental resistance of the blade. Anadditional advantage of the present invention is that the benefits ofthe blade tip alloy and the method by which the blade tip is formed areapplicable to both new and worn turbine blades. Finally, the preferredtechnique by which the blade tip is generated further promotes a strongweld, so as to maximize the overall mechanical properties of the turbineblade.

While our invention has been described in terms of a preferredembodiment, it is apparent that other forms could be adopted by oneskilled in the art. Therefore, the scope of our invention is to belimited only by the following claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A turbine blade for agas turbine engine, the turbine blade having a blade tip welded thereto,the blade tip being formed from an alloy consisting, in weight percent,essentially of:about 15 to about 18 percent chromium; about 6.5 percentaluminum; about 10 percent cobalt; about 6 percent tantalum; about 2percent rhenium; about 0.05 to about 1.5 percent hafnium; about 0.015 toabout 0.03 percent zirconium; about 0.02 to about 0.1 percent carbon; upto about 1 percent silicon; up to about 0.01 percent boron; with thebalance being nickel and typical impurities, and being substantiallyfree of yttrium.
 2. The turbine blade of claim 1 wherein the remainderof the turbine blade is formed from an alloy, and wherein the alloy ofthe blade tip differs from that of the remainder of the turbine blade.3. A turbine blade for a gas turbine engine, the turbine blade having ablade tip welded thereto, the blade tip being formed from an alloyconsisting, in weight percent, essentially of:about 15 to about 17percent chromium; about 6.45 to about 6.95 percent aluminum; about 9.75to about 11.25 percent cobalt; about 5.95 to about 6.45 percenttantalum; about 1.85 to about 2.25 percent rhenium; about 1.45 to about1.75 percent hafnium; about 0.008 to about 0.028 percent zirconium;about 0.02 to about 0.04 percent carbon; about 0.7 to about 1.1 percentsilicon; about 0.003 to about 0.007 percent boron; with the balancebeing nickel and typical impurities.
 4. A turbine blade for a gasturbine engine, the turbine blade having a blade tip welded thereto, theblade tip being formed from an alloy consisting, in weight percent,essentially of:about 14.6 to about 16.6 percent chromium; about 6.45 toabout 6.95 percent aluminum; about 9.95 to about 11.45 percent cobalt;about 6.05 to about 6.55 percent tantalum; about 1.95 to about 2.35percent rhenium; about 1.45 to about 1.75 percent hafnium; about 0.008to about 0.028 percent zirconium; about 0.09 to about 0.11 percentcarbon; up to about 0.2 percent silicon; up to about 0.002 percentboron; with the balance being nickel and typical impurities.